1. Field of the Invention
The invention relates to a method of generating an approximately sinusoidal signal from triangular signals and to a circuit arrangement for implementing this method.
2. Description of the Prior Art
Such a method is known in essential from the book by Graeme, Toby and Huelsman: "Operational Amplifiers in Nonlinear Circuits" McGraw-Hill, 1971, page 259, FIG. 7.27a. Here, a diode function generator, which has parts of the characteristic with different gradient, is used to generate from a triangular signal a period signal which has a certain similarity with a sine-wave oscillation. One disadvantage of the known method lies in the fact that the approximately sinusoidal output signal of the diode function generator changes not only its amplitude but also its shape when the amplitude of the supplied triangular signal varies.
Sinusoidal signals can in fact be generated by means of suitable sine-wave oscillators, but such oscillators require either suitable oscillating circuit elements, e.g. quartz crystals, or inductances and cannot therefore be produced as an integrated circuit at a reasonable cost. On the other hand, however, particularly in the case of stereo demodulators, there is a desire for a signal with the frequency of the pilot sound or of the subcarrier which is as sinusoidal as possible.
Originally, square-wave signals with the desired frequency were used, but these deviated a relatively large amount from the sinusoidal. In particular, the third harmonic of the fundamental frequency contained therein considerably impairs the properties of a demodulator using these oscillations. DE-OS 27 39 668 describes the use of a triangular signal whose third harmonic is smaller by a factor of three than that of a square-wave signal in place of a square-wave signal. Finally, U.S. Pat. No. 4,300,019 describes the generation of a three-stage, periodic square-wave signal which is formed in such a way that no third harmonics occur. The cost of generating such a signal is, of course, relatively high, and in some applications, for example when a sinusoidal (pilot) signal is to be compensated by another signal, it is not simply a matter of the third harmonic disappearing, but is also important that its variation with time corresponds as exactly as possible with that of a sinusoidal signal.